VCLT

The Vienna Convention on the Law of Treaties was adopted in Vienna on 23 May 1969 and entered into force on 27 January 1980 after reaching 35 ratifications. It is often called the "treaty on treaties" because it sets out the rules governing the conclusion, interpretation, modification, suspension, and termination of treaties between states.

Key provisions delegates and researchers cite most often include:

• Article 2 — defines a "treaty" as an international agreement concluded between states in written form and governed by international law.
• Articles 6–18 — cover capacity, full powers, consent to be bound, signature, ratification, and reservations.
• Article 26 — codifies pacta sunt servanda: treaties in force are binding and must be performed in good faith.
• Article 27 — a party may not invoke its internal law to justify failure to perform a treaty.
• Articles 31–33 — the general rule of interpretation (ordinary meaning, context, object and purpose), supplementary means including travaux préparatoires, and rules for treaties in multiple languages.
• Article 53 — voids any treaty conflicting with a peremptory norm of general international law (jus cogens).
• Article 60 — material breach as a ground for termination or suspension.
• Article 62 — rebus sic stantibus, or fundamental change of circumstances.

The VCLT applies only to treaties concluded after its entry into force for the parties concerned and only to treaties between states — a separate 1986 Vienna Convention covers treaties involving international organizations (not yet in force). Notable non-parties include the United States, France, and India, though the U.S. State Department treats most substantive provisions as reflecting customary international law, a view shared by the International Court of Justice in cases such as Gabčíkovo-Nagymaros (Hungary/Slovakia, 1997).